The present invention relates to a process for coating a prosthesis made of titanium or a titanium alloy, and more particularly relates to coating such a prosthesis by sinter-fusing coating.
Currently endoprostheses are mostly fixed with bone cement into the prepared prosthesis position. This method of attachment has recently encountered more and more criticism, as the system "metallic prosthesis/bone cement/bone" does not have sufficient durability, with the consequences that the prostheses implanted in this manner loosen.
For this reason it has been tried for some time to fix prostheses in the bone without cement. In order to attain in this case a good contact between bone and implant, efforts are made to produce prostheses with a structured surface. Such prostheses can basically be produced by lost-wax casting or by sinter coating an already shaped prosthesis with particles made from the same type of material as the already shaped prosthesis to provide a grainy coating. In the case of prostheses of titanium alloys the process is, as now known, to sinter-fuse particles onto a forged prosthesis or onto a prostheses produced by a chip removal process.
The disadvantage with this process is that a sufficient fastening of, e.g., titanium spheres or balls onto a titanium shaft by sintered heat treatment can only be attained at temperatures clearly above the .alpha.-.beta.-transition temperature. A coating above this temperature has the effect that the working material (the material of the shaft) becomes coarse grained and looses the good mechanical properties which it possessed in its starting condition before the sinter-fusing.
Thus, there are described, e.g., in St.A. Cook, "Fatigue properties of carbon- and porous-coated Ti-6Al-4V alloy", Journal of Biomedical Materials Research, Volume 18, 2. 497 to 512, 1984, investigations of the bending fatigue endurance limit of untreated and coated samples of the titanium alloy Ti-6Al-4V, often used as implant material, proving that the bending fatigue endurance limit of the untreated titanium alloy of 617 N/mm.sup.2 drops with coating by sintered heat treatment to 138 N/mm.sup.2. The drop in endurance can be attributed to the superposition of two effects: on the one hand, the coarsening of the grain by heat treatment above the transition temperature and, on the other hand, the surface structure which improves the notch stress. For this reason, samples were also tested which were subjected to the temperature cycle of the heat treatment of the coating process, but were not coated. In these tests a bending fatigue endurance limit of 377 N/mm.sup.2 was determined. This proves that a large part of the reduction in bending fatigue endurance limit of titanium alloys can be attributed to the coarsening of the grain by the coating process executed above the transition temperature.